The morphological changes occurring in monkey luteal cells in culture were correlated with the concomitant decline in steroidogenic activity. The results showed that under our culture conditions, despite the presence of hCG, the luteal cells gradually lost their original morphological appearance and assumed a new flattened phenotypic appearance. These changes in cell shape paralleled observations on declining progesterone synthesis by luteal cells in culture. In a separate study we demonstrated that horseradish peroxidase conjugated hCG (HRP:hCG) when purified binds specifically to LH/hCG receptors of incubated monkey luteal cells. Upon binding with the receptor the HRP:hCG is internalized by the luteal cells.